A mode converter is a photonic device that is configured to convert an optical mode between a first mode size and a second mode size. Mode size refers to the dimension of a mode in an optical waveguide in a certain direction, for example, the energy distribution in the transverse direction. Mode shape refers to the relative dimension of the mode size in two different directions, for example, a horizontal direction and a vertical direction. Due to the small mode size (e.g., sub-micro) of silicon waveguides it is challenging for mode converters to couple light into and/or out of silicon-based photonic devices. An existing method transfers the optical mode in a silicon waveguide to a large-size (e.g., with a mode diameter from about 1 micrometer (μm) to about 15 μm) low-index (e.g., an index from about 1 to about 3.4) waveguide using silicon inverse taper structures to increase a mode size. Another method uses inverse taper structures to increase the mode size by expanding the mode to a low-index cladding. However, these methods require a silicon waveguide with a small tip. Creating a tip width that is less than 100 nanometer (nm) in a standard 200 millimeter (mm) fabrication process is difficult due to lithography limitations. Current methods for creating a silicon waveguide with a small tip include using vertical tapering and knife-edge tapering. However, these silicon waveguides suffer from complicated and a non-controllable process.